Gas generant composition

ABSTRACT

Preferred gas generant compositions incorporate a combination of 5-aminotetrazole nitrate and an oxidizer. The oxidizer may be selected from a group including nonmetal and metal nitrates, nitrites, chlorates, chlorites, perchlorates, and oxides. 5-aminotetrazole nitrate is characterized as an oxygen-rich fuel and is therefore considered to be a self-deflagrating fuel. To tailor the oxygen balance in certain applications, however, the use of an oxidizer is preferred. Methods of formulating the compositions are also described. These compositions are especially suitable for inflating air bags and actuating seatbelt pretensioners in passenger-restraint devices.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of copending U.S.application Ser. No. 09/544,694 filed on Apr. 7, 2000 that is acontinuation-in-part of application Ser. No. 09/516,067, now U.S. Pat.No. 6,287,400, filed on Mar. 1, 2000.

BACKGROUND OF THE INVENTION Field of the Invention

[0002] The present invention relates to nontoxic gas generatingcompositions that when combusted rapidly generate gases that are usefulfor actuating vehicle occupant restraint systems in motor vehicles andspecifically, the invention relates to thermally stable nonazide gasgenerants having not only acceptable burn rates and sustainedcombustion, but also a relatively high gas volume to solid particulateratio at acceptable flame temperatures.

[0003] The evolution from azide-based gas generants to nonazide gasgenerants is well-documented in the prior art. The advantages ofnonazide gas generant compositions in comparison with azide gasgenerants have been extensively described in the patent literature. Seefor example, U.S. Pat. Nos. 4,370,181; 4,909,549; 4,948,439; 5,084,118;5,139,588, 5,035,757, 5,386,775, and 5,872,329, the discussions of whichare hereby incorporated by reference.

[0004] In addition to a fuel constituent, pyrotechnic nonazide gasgenerants often contain ingredients such as an oxidizer to provide therequired oxygen for rapid combustion and reduce the quantity of toxicgases generated, a catalyst to promote the conversion of toxic oxides ofcarbon and nitrogen to innocuous gases, and a slag forming constituentto cause the solid and liquid products formed during and immediatelyafter combustion to agglomerate into filterable clinker-likeparticulates. Other optional additives, such as burning rate enhancersor ballistic modifiers and ignition aids, are used to control theignitability and combustion properties of the gas generant.

[0005] One of the disadvantages of known nonazide gas generantcompositions is the amount and physical nature of the solid residuesformed during combustion. The solids produced as a result of combustionmust be filtered and otherwise kept away from contact with the occupantsof the vehicle. It is therefore highly desirable to develop compositionsthat produce a minimum of solid particulates while still providingsufficient quantities of a nontoxic gas to inflate the safety device atan acceptable rate.

[0006] The use of phase stabilized ammonium nitrate is desirable becauseit generates abundant nontoxic gases and minimal solids upon combustion.To be useful, however, gas generants for automotive applications must bethermally stable.

[0007] Often, gas generant compositions incorporating phase stabilizedor pure ammonium nitrate exhibit poor thermal stability, and produceunacceptably high levels of toxic gases, CO and NO_(x) for example,depending on the composition of the associated additives such asplasticizers and binders. In addition, ammonium nitrate contributes topoor ignitability, lower burn rates, and performance variability.Several known gas generant compositions incorporating ammonium nitrateutilize well-known ignition aids such as BKNO₃ to solve this problem.However, the addition of an ignition aid such as BKNO₃ is undesirablebecause it is a highly sensitive and energetic compound, andfurthermore, contributes to thermal instability and an increase in theamount of solids produced.

[0008] Certain gas generant compositions comprised of ammonium nitrateare thermally stable, but have burn rates less than desirable for use ingas inflators. To be useful for passenger restraint inflatorapplications, gas generant compositions generally require a burn rate ofat least 0.4 inch/second (ips) or more at 1000 psi. Gas generants withburn rates of less than 0.40 ips at 1000 psi do not ignite reliably andoften result in “no-fires” in the inflator.

[0009] Yet another problem that must be addressed is that the U.S.Department of Transportation (DOT) regulations require “cap testing” forgas generants. Because of the sensitivity to detonation of fuels oftenused in conjunction with ammonium nitrate, most propellantsincorporating ammonium nitrate do not pass the cap test unless shapedinto large disks, which in turn reduces design flexibility of theinflator.

[0010] The compositions described in U.S. Pat. No. 5,035,757 to Pooleexemplify state of the art gas generant compositions that function wellbut produce relatively large amounts of solid combustion products. As aresult, the gas produced is less than that produced by current state ofthe art “smokeless” gas generants. Thus, more gas generant and greaterfiltering requirements are required to facilitate operation of an airbaginflator.

[0011] On the other hand, compositions described in U.S. Pat. No.5,872,329 to Burns et al. exemplify current state of the art “smokeless”gas generants. The combustion products are primarily gas with minimalformation of solids. The benefits include a reduction in the amount ofgas generant required and reduced filtering requirements. However,certain compositions described by Burns may be disadvantaged by lowerburn rates and a failure to sustain gas generant combustion. To overcomethese disadvantages, a stronger and more robust inflator is oftenrequired to increase the operating pressure of the inflator and therebyimprove the burn of the gas generant.

[0012] Accordingly, it would be an improvement in the art to provide thegas generant burn characteristics of compounds as described in U.S. Pat.No. 5,035,757 along with the capacity to produce more gas and lesssolids as typified by state of the art “smokeless” gas generants.

SUMMARY OF THE INVENTION

[0013] The present invention generally relates to gas generantcompositions useful in actuating a vehicle occupant restraint system inthe event of a motor vehicle accident. Application within a vehicleoccupant restraint system includes actuation of a seatbelt pretensionerand/or inflation of an airbag. Other applications requiring gasgeneration are also contemplated, including fire suppression systemsaboard aircraft and inflators for flotational devices, for example.

[0014] The above-referenced problems are reconciled by compositionscontaining 5-aminotetrazole nitrate (5ATN) as a fuel at about 25-100% byweight of the total composition. An oxidizer is selected from a group ofcompounds including phase stabilized ammonium nitrate, ammonium nitrate,potassium nitrate, strontium nitrate, copper dioxide, and basic coppernitrate. Other oxidizers well known in the art are also contemplated.These generally include but are not limited to inorganic oxidizers suchas alkali and alkaline earth metal nitrates, nitrites, chlorates,chlorites, perchlorates, and oxides.

[0015] Standard binders, slag formers, and coolants may also beincorporated if desired.

[0016] A composition in accordance with the present invention containsby weight 25-95% 5ATN and 5-75% of an oxidizer. A more preferredcomposition consists of 55-85% 5ATN and 20-45% PSAN.

[0017] A method of formulating compositions of the present inventionincludes providing an excess amount of nitric acid (preferably 15.9M orless and preferably chilled at 0-20° C.), and then, in the appropriateamounts, adding a nitratable fuel such as 5-aminotetrazole and at leastone oxidizer to the nitric acid. The slurry is stirred until a damp orwet paste forms. The paste is then formed into the desired shapes anddried.

BRIEF DESCRIPTION OF THE FIGURES

[0018]FIG. 1 illustrates the burn rate of state of the art “smokeless”gas generants as compared to a preferred embodiment of the presentinvention.

[0019]FIG. 2 illustrates the 60L tank pressure and chamber pressureresulting from combustion of state of the art “smokeless” gas generantsand a preferred embodiment of the present invention.

[0020]FIG. 3 illustrates a comparison of pressure vs. time in a 40 cctank with respect to state of the art compositions, preferredembodiments of the present invention and control compositions.

[0021]FIG. 4 illustrates the melting point and decompositiontemperatures of a preferred embodiment of the present invention, as wellas related data separately comparing the respective constituents of thepreferred embodiment.

[0022]FIG. 5 illustrates the autoignition temperature of a preferredembodiment of the present invention.

[0023]FIG. 6 illustrates the infrared scans of 5-AT, AN, KN, and the5-AT.HNO₃/PSAN10 mixture. The presence of strong nitrate peaks andshifts in the N-H peaks affirms the formation of 5-AT.HNO₃ when thecomposition is formulated as described herein.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] The gas generants of the present invention when compared to otherstate of the art gas generants ignite easier, produce minimal solids,exhibit improved burn rates, are thermally stable, and sustain a burn atlower pressures.

[0025] In accordance with the present invention, 5-aminotetrazolenitrate (5-ATN) is provided at 25-100% by weight of the gas generant,depending on the application. 5-ATN is characterized as an oxygen-richfuel attributed to the oxygen in the nitrate group. The use of 5-ATNwithin a gas generant composition therefore requires little or noadditional oxidizer, again depending on the application. 5-ATN is morepreferably provided at 30-95% by weight and most preferably provided at55-85% by weight of the gas generant composition.

[0026] In certain applications, the oxygen balance must be tailored toaccommodate reduced levels of carbon monoxide (CO) and nitrogen oxides(NOx) as driven by original equipment manufacturer toxicityrequirements. For example, the gas generated upon combustion of a gasgenerant within a vehicle occupant restraint system must minimize oreliminate production of these toxic gases. Therefore, when adding anoxidizer to 5-ATN, it is generally understood that an oxygen balance ofabout −4.0 to +4.0 is desirable when the gas generant is used in anairbag inflator. The preferred percentages of 5-ATN reflect thischaracteristic.

[0027] One or more oxidizers may be selected from the group includingnonmetal, alkali metal, and alkaline earth metal nitrates, nitrites,perchlorates, chlorates, and chlorites for example. Other oxidizers wellknown in the art may also be used. These include alkali, alkaline earth,and transitional metal oxides, for example. Preferred oxidizers includephase stabilized ammonium nitrate (PSAN), ammonium nitrate, potassiumnitrate, and strontium nitrate. The oxidizer(s) is provided at 5-70% byweight of the gas generant composition and more preferably at 20-45% byweight of the oxidizer.

[0028] Standard additives such as binders, slag formers, burn ratemodifiers, and coolants may also be incorporated if desired. Inertcomponents may be included and are selected from the group containingclay, silicon, silicates, diatomaceous earth, and oxides such as glass,silica, alumina, and titania. The silicates include but are not limitedto silicates having layered structures such as talc and the aluminumsilicates of clay and mica; luminosilicate; borosilicates; and othersilicates such as sodium silicate and potassium silicate. The inertcomponent is present at about 0.1-20% by weight, more preferably atabout 0.1-8%, and most preferably at 0.1-3%.

[0029] A most preferred embodiment contains 73.12% 5-ATN and 26.88%PSAN10 (ammonium nitrate stabilized with 10% potassium nitrate). Theinvention is further exemplified by the following examples.

EXAMPLE 1

[0030] 5ATN was prepared according to the following method. In an icebath, 20 g (0.235 moles) of anhydrous 5-aminotetrazole and 22 ml (0.350moles) of concentrated nitric acid were stirred for about one hour.About 70 ml of water was added directly to the slurry, and the entiremixture was heated quickly to boiling. The hot solution was vacuumfiltered and allowed to cool at ambient conditions while stirring. Thewhite crystals formed during cooling were vacuum filtered and washedwith cold water, then forced through a No. 14 mesh screen to formgranules. The wet material was dried for one hour at ambient conditionsand formed well-flowing granules.

[0031] As determined by TGA, the 5ATN dried at ambient conditionscontained about 1.0 wt % water. As tested on a BOE impact apparatus,this material showed no positive fires up to 25 inches (equivalent toabout 231 kp.cm).

EXAMPLE 2

[0032] The 5ATN granules prepared in Example 1 were dried at 105 degreesCelsius for 4 hours to remove any remaining moisture. Elemental analysisfor C, H, and N showed by weight 8.36% carbon, 2.71% hydrogen, and56.71% nitrogen. The theoretical values by weight are 8.11% C, 2.72% H,56.75% N, and 32.41% O.

[0033] As tested on a BOE impact apparatus, this material showedpositive fires at about 4 inches (equivalent to about 37 kp.cm). Thisdemonstrates how 5ATN experiences an increase in impact sensitivity whencompletely dry.

[0034] The dried 5ATN was tested using a DSC at a heating rate of 10degrees Celsius per minute. The 5ATN melted at 156.8 degrees Celsius andthen decomposed exothermically with an onset of 177.2 degrees Celsiusand a peak of 182.5 degrees Celsius. The 5ATN was also tested using aTGA at a heating rate of 10 degrees Celsius per minute and found to havean 89.3 wt. % gas conversion up to 450 degrees Celsius, with a 67.5 wt.% gas conversion up to about 194 degrees Celsius. The DSC and TGA datashow that the 5ATN autoignites at about 180 degrees Celsius with a largerelease of energy.

EXAMPLE 3

[0035] The wet 5ATN granules as prepared in Example 1 were compressionmolded in a 0.5 inch die under a 10-ton force to a height of about 0.1inches. About half of the pellets were dried for 4 hours at 70 degreesCelsius to remove all the moisture. A weight loss of about 1.0 wt. %confirmed that all of the moisture had been removed.

[0036] Both the wet and dry 5ATN pellets were tested as a boostermaterial using the following specifications. Each pellet was broken intofour pieces and the fragments were loaded into a small aluminum cup.This aluminum cup was then crimped to a standard air bag initiator thatcontained 110 mg of zinc potassium perchlorate (ZPP). The entireassembly, known as an igniter, was fired inside a closed bomb with avolume of 40 cubic centimeters. The 40 cubic centimeter bomb wasequipped with a pressure transducer to measure the pressure rise overtime.

[0037]FIG. 3 shows the results of the tests. Other control tests weredone as a comparison to the igniters containing 5ATN. Tests 7 and 8(initiator) are igniters consisting of an empty aluminum cup crimped toan initiator. Tests 9 and 10 are control igniters containing both anautoignition material and 8 pellets of a standard nonazide compositionas described in U.S. Pat. No. 5,035,757. Test 11 is another controligniter similar to tests 9 and 10, except with autoignition material and14 pellets of the same nonazide composition. Test 12 is an ignitercontaining about 0.7 g of undried 5ATN pellet fragments Test 13 is anigniter containing about 1.0 g of undried 5ATN pellet fragments. In allcases, the igniters containing 5ATN ignited readily and actually reachedpeak pressure sooner than the control igniters. In both tests 11 and 13,the volume of the aluminum cup was completely full. As shown in FIG. 3,for an equivalent volume of material, the output of the 5ATN igniter(13) is about twice that of the control igniter containing the state ofthe art propellant (11).

EXAMPLE 4

[0038] A composition was prepared containing 77.77 wt. % 5ATN and 22.23wt. % strontium nitrate. The 5ATN as prepared in Example 1 and driedstrontium nitrate were combined to form an overall mass of 0.71 g andthen mixed and ground with a mortar and pestle. The composition wastested by DSC at a heating rate of 5 degrees Celsius per minute andfound to melt at 155.3 degrees Celsius and then decomposed with a largeexotherm (175.6 degrees Celsius onset, 179.4 degrees Celsius peak). Thecomposition was tested by TGA at a heating rate of 10 degrees Celsiusper minute and found to have a 91.7 wt. % gas conversion up to 950degrees Celsius, with a 74.1 wt. % gas conversion up to about 196degrees Celsius. As tested on a BOE impact apparatus, this compositionshowed positive fires at about 3 inches (equivalent to about 28 kp.cm).This composition burned vigorously when ignited with a propane torch.

EXAMPLE 5

[0039] A composition was prepared containing 65.05 wt. % 5ATN and 34.95wt. % copper (II) oxide. The 5ATN as prepared in Example 1 and the driedcopper oxide were combined to form an overall mass of 0.52 g and thenmixed and ground with a mortar and pestle. The composition was tested byDSC at a heating rate of 10 degrees Celsius per minute and found todecompose with a large exotherm peaking at about 175 degrees Celsius.The composition was tested by TGA at a heating rate of 10 degreesCelsius per minute and found to have an 83.4 wt. % gas conversion up to400 degrees Celsius, with an 80.6 wt. % gas conversion up to about 183degrees Celsius. As tested on a BOE impact apparatus, this compositionshowed positive fires at about 3 inches (equivalent to about 28 kp.cm).This composition burned vigorously when ignited with a propane torch.

EXAMPLE 6

[0040] A composition was prepared containing 65.05 wt. % 5ATN and 34.95wt. % copper (II) oxide. The 5ATN as prepared in Example 1 and the driedcopper oxide were combined to form an overall mass of 1.00 g. Enoughwater was added to form a slurry and then the components were mixed andground with a mortar and pestle. The water was allowed to evaporate byholding the mixture at 70 degrees Celsius. Eventually, a sticky,polymer-like substance formed which became very hard with completedrying. The composition was tested by DSC at a heating rate of 10degrees Celsius per minute and found to exhibit multiple exothermsbeginning at about 137 degrees Celsius. This composition burnedvigorously when ignited with a propane torch. This example demonstrateshow 5ATN can be combined with a common oxidizer through either dry orwet mixing.

EXAMPLE 7

[0041] A composition was prepared containing 67.01 wt. % 5ATN and 32.99wt. % PSAN10 (AN phase stabilized with 10 wt. % KN). The 5ATN asprepared in Example 1 and the dried PSAN10 were combined to form anoverall mass of 0.24 g and then mixed and ground with a mortar andpestle. FIG. 5 shows a melting point of 132° C. and a decompositionpoint of 153° C. See curve 17. The various constituents are alsoanalyzed separately. See curves 14-16.

EXAMPLE 8

[0042] As FIG. 1 illustrates, gas generants of the present invention asexemplified by curve 1, have acceptable burn rates at ambient pressuresand above, and have significantly higher burn rates as compared to stateof the art “smokeless” gas generants (curve 2). Curve 1 indicates a gasgenerant containing 73.12% 5-ATN and 26.88% phase stabilized ammoniumnitrate (stabilized with 10% potassium nitrate). Curve 2 is a comparisonof a gas generant containing 65.44% phase stabilized ammonium nitrate(stabilized with 10% potassium nitrate or PSAN10), 25.80% of thediammonium salt of 5,5′-Bi-1H-tetrazole, 7.46% strontium nitrate, and1.30% clay. The pressure exponent of the present invention, 0.71 is lessthan the pressure exponent of the state of the art “smokeless” gasgenerant of curve 2, 0.81. As shown in FIG. 2, a typical embodimentautoignites at 147° C. See curve 21. The gas generant constituents whentaken alone do not indicate autoignition from 0-400° C.

EXAMPLE 9

[0043]FIG. 3 illustrates a comparison between a preferred embodimentcontaining the same fuel as curve 1 in Example 8. See curves 3 and 4.Curves 5 and 6 correspond to the same “smokeless” gas generant asindicated in curve 2 of Example 8. As curves 3 and 5 indicate, thechamber pressure resulting from combustion of the preferred embodimentis at 26 Mpa whereas the chamber pressure of the state of the art“smokeless” gas generant is 37 Mpa. On the other hand, as shown incurves 4 and 6, the 60L tank pressures are approximately equivalentgiven the same inflator. The data can be interpreted to show thatcompositions of the present invention require less pressure but maintainsuperior burn rates (see FIG. 1) and thus are able to provideapproximately equivalent inflation pressure for an airbag. As a result,a less robust inflator with a weaker ignition source may be used incompositions of the present invention. Compare the igniters used in FIG.3 and Example 3.

EXAMPLE 10

[0044] Two compositions were prepared and tested. The burn rate wasmeasured by igniting a compressed slug in a closed bomb at a constantpressure of 1000 psi. The ignitability of the formulations wasdetermined by attempting to ignite the samples at ambient pressure witha propane torch. The outputs of the subjective analysis are thefollowing: the time it takes for the sample to reach self-sustainingcombustion after the torch flame touches the sample, and the ease ofwhich the sample continues combustion when the torch flame is removed.

[0045] Formulation 1 was 73.12% 5-ATN and 26.89% PSAN10. The sampleignited instantly when touched with the flame from a propane torch andcontinued to burn vigorously when the flame was removed. The burn rateof this formulation at 1000 psi was measured to 0.69 inches per second(ips). To minimize the production of either CO or NOx, this compositionwas formulated to have an oxygen balance of −2.0 wt. % oxygen.

[0046] Formulation 2 was 62.21% azobisformamidine dinitrate and 37.79%PSAN10. When contacted with the flame from a propane torch, the sampledid not ignite for a few seconds. After it appeared that self-sustainingcombustion had begun, the torch was removed and the sample extinguished.After igniting the sample a second time, it burned slowly to completion.The burn rate of this formulation at 1000 psi was measured at 0.47 ips.To minimize the production of either CO or NOx, this composition wasformulated to have an oxygen balance of 0.0 wt. % oxygen.

[0047] It is believed that the nitrated 5-AT fuel ignites more easilyand burns faster for the following reasons:

[0048] 1) The base 5-AT fuel has more energy (positive heat offormation) than the base azobisformamidine fuel (negative heat offormation).

[0049] 2) The nitrated 5-AT has a higher oxygen content and thereforeallows for the use of a lesser amount of the PSAN oxidizer. It is wellknown that the higher levels of PSAN will negatively affect theignitability and burn rate of many propellant compositions.

EXAMPLE 11

[0050] Table 1 illustrates the problem of thermal instability whentypical nonazide fuels are combined with PSAN: Nonazide Fuel(s) Combinedwith PSAN Thermal Stability 5-aminotetrazole (5AT) Melts with 108 C.onset and 116 C. peak. Decomposed with 6.74% weight loss when aged at107 C. for 336 hours. Poole ′272 shows melting with loss of NH₃ whenaged at 107 C. Ethylene diamine Poole ′272 shows melting at less than100 C. dinitrate, nitroguanidine (NQ) 5AT, NQ Melts with 103 C. onsetand 110 C. peak. 5AT, NQ quanidine nitrate Melts with 93 C. onset on 99C. peak. (GN) GN, NQ Melts with 100 C. onset and 112 C. Decomposed with6.49% weight loss when aged at 107 C. for 336 hours. GN,3-nitro-1,2,4-triazole Melts with 108 C. onset and 110 C. peak. (NTA)NQ, NTA Melts with 111 C. onset and 113 C. peak. Aminoguanidine nitrateMelts with 109 C. onset and 110 C. peak. 1H-tetrazole (1 HT) Melts with109 C. onset and 110 C. peak. Dicyandiamide (DCDA) Melts with 114 C.onset and 114 C. peak. GN, DCDA Melts with 104 C. onset and 105 C. peak.NQ, DCDA Melts with 107 C. onset and 115 C. peak. Decomposed with 5.66%weight loss when aged at 107 C. for 336 hours. 5AT, GN Melts with 70 C.onset and 99 C. peak. Magnesium salt of 5AT Melts with 100 C. onset and111 C. peak.

[0051] In Example 11, “decomposed” indicates that pellets of the givenformulation were discolored, expanded, fractured, and/or stuck together(indicating melting), making them unsuitable for use in an air baginflator. In general, any PSAN-nonazide fuel mixture with a meltingpoint of less than 115C. will decompose when aged at 107C. As shown,many compositions that comprise well-known nonazide fuels and PSAN arenot fit for use within an inflator due to poor thermal stability. Asshown in FIG. 4 curve 17, the melting point of a preferred embodiment isgreater than 115C. (132C.), thereby indicating that combining 5-ATN withPSAN does not significantly affect the stability of the propellant.

EXAMPLE 12

[0052] A composition containing 73.12% 5-ATN and 26.88% PSAN10 has beentested for sensitivity with the following results: Impact (BOEApparatus) 48 kp · cm Friction (BAM Apparatus) 120 N ElectrostaticDischarge >900 mJ

[0053] The preferred composition was compared to nitrocellulose, astandard gas generant for seat belt pretensioners. Gas yield, gasconversion, autoignition temperature, solids production, combustiontemperatures, and density were roughly equivalent. Seat belt retractortests also revealed fairly equivalent performance results. The followingdata was developed relative to nitrocellulose: Impact (BOE Apparatus) 29kp · cm Friction (BAM Apparatus) >360 N Electrostatic Discharge NA

[0054] The preferred embodiment resulted in combustion gases containing0.0% CO and 2.4% hydrogen, and 97.6% preferred gases containingnitrogen, carbon dioxide, and water. On the other hand, nitrocelluloseresulted in combustion gases containing 29.2% CO and 19.7% hydrogen, and51.1% preferred gases containing nitrogen, carbon dioxide, and water.

[0055] It can therefore be concluded that compositions of the presentinvention provide similar performance to nitrocellulose but withimproved thermal stability, impact sensitivity, and content of effluentgases when used as a pretensioner gas generant.

EXAMPLE 13

[0056] Compositions containing 100% 5-ATN were used as pretensioner gasgenerants despite exhibiting an oxygen balance of −10.80wt. % oxygen.The amount of gas generant used in a pretensioner is small enough(roughly one gram) to permit an excessive negative oxygen balancewithout prohibitive levels of CO.

EXAMPLE 14

[0057] As shown in Table 2, other compositions of the present inventioninclude gas generants exhibiting oxygen balances in the range of −11.0to +11.0. The oxygen balance may be readily determined by well knowntheoretical calculations. An oxygen balance of about +4.0 to −4.0% ispreferred for compositions used in vehicle occupant restraint systems asmain gas generants. Compositions exhibiting an oxygen balance outside ofthis range are useful as autoignition compounds or igniter compounds inan inflator; as a pretensioner gas generant; in a fire suppressionmechanism; as a gas generant for an inflatable vessel or airplane ramp,or where levels of toxic gases such as CO and NOx are not critical forthe desired use. TABLE 2 Gas Gas Yield Conversion Gas Oxygen (moles/ (wt% Products Balance Composition 100 g) to gas) (vol. %) (wt % O2) Example4 3.26 89.1 51.6% N2 0.0 32.3% H2O 16.1% CO2 Example 5 2.64 72.1 50.0%N2 0.0 33.3% H2O 16.7% CO2 35% 5-ATN 3.91 98.1 42.3% N2 −2.16 41% PSAN1047.5% H2O 24% NQ 10.0% CO2 39.4% 5-ATN 3.95 97.2 38.2% N2 +9.06 60.6%PSAN10 47.9% H2O 6.7% CO2 7.2% O2 73.1% 5-ATN 3.82 98.8 46.4% N2 −2.026.9% PSAN10 38.4% H2O 11.9% CO2 2.4% H2 60.0% 5-ATN 3.87 98.1 43.5% N2+2.3 40.0% PSAN10 44.2% H2O 10.5% CO2 1.8% O2 79.2% 5-ATN 3.80 99.048.7% N2 −4.0 20.8% PSAN10 37.2% H2O 14.1% CO2

[0058] The oxygen balance is the weight percent oxygen necessary toresult in stoichiometric combustion of the propellant. 5-aminotetrazolenitrate has a less negative oxygen balance than typical nonazide fuelsand is considered to be self-deflagrating. This allows for compositionswith significantly less PSAN (or other oxidizer) which will ignite morereadily and combust at lower inflator operating pressures thanpreviously known smokeless gas generants. Essentially, thesecompositions combine the benefits of the typical high-solids nonazidegas generants as exemplified by U.S. Pat. No. 5,035,757 to Poole (highburn rate, easily ignitable, low inflator operating pressures) with thebenefits of PSAN-based smokeless nonazide gas generants exemplified inU.S. Pat. No. 5,872,329 to Burns et al. (90-100% gas conversion, minimalsolids). The result is an inflator that is smaller, lighter, cheaper andless complex in design. Other well-known gas generant constituents mayalso be used in accordance with the present invention. See thosedescribed in the Background of the Invention, for example.

[0059] In yet another aspect of the invention, methods of formulatinggas generant compositions containing 5-ATN, or any other nitrated basefuel, are described. The nitratable base fuels (i.e. the base fuelsprior to nitration) include, but are not limited to nitrourea,5-aminotetrazole, diaminotriazole, urea, azodicarbonamide,hydrazodicarbonamide, semicarbazide, carbohydrazide, biuret,3,5-diamino-1,2,4-triazole, dicyandiamide, and 3-amino-1,2,4-triazole.Each of these base fuels may be nitrated and combined with one or moreoxidizers. Thus, methods of forming gas generant compositions containing5-ATN and one or more oxidizers, as described below but not therebylimited, exemplify the manufacture of gas generant compositionscontaining any nitrated base fuel and one or more oxidizers.

[0060] The constituents of the gas generant compositions may all beobtained from suppliers well known in the art. In general, the base fuel(5AT) and at least one oxidizer are added to excess concentrated nitricacid and stirred until a damp paste forms. This paste is then formedinto granules by either extrusion or forcing the material through ascreen. The wet granules are then dried. It has been found that theprocess not only forms a nitrated fuel, but also forms particularlyintimate mixtures when the oxidizer is added in solution. The crystalsformed thus represent homogeneous 5-AT nitrate/oxidizer solid solutions.This is particularly advantageous when homogeneous granules are desiredbecause the probability of inconsistent mixing on the granular level issubstantially reduced. Stated another way, the granules formed from thesolid solution actually represent homogeneous solutions whereas a givengranule formed from dry mixing, for example, at times may primarilycomprise either the fuel or oxidizer, but not both. The performance andburn rate can therefore be disadvantaged.

[0061] The process also comprises a “one-pot” process. For example, if acomposition containing 5-AT nitrate and PSAN is desired, then combining5-AT, ammonium nitrate and potassium nitrate in a concentrated nitricacid solution results in a composition containing 5-AT nitrate and PSAN.Thus, two different processes are not required to form both the 5-ATnitrate and the PSAN, and yet a composition enjoying the inherentbenefits of both results. Related benefits include simplified processingand a reduction in manufacturing costs.

[0062] The nitric acid can be the standard reagent grade (15.9M,-70 wt.% HNO₃) or can be less concentrated as long as enough nitric acid ispresent to form the mononitrate salt of 5AT. The nitric acid shouldpreferably be chilled to 0-20° C. before adding the 5AT and oxidizers toensure that the 5AT does not decompose in the concentrated slurry.However, shortening the process time will also inhibit the decompositionof 5AT. When mixing the 5AT and oxidizers in the nitric acid medium, theprecise mixing equipment used is not important—it is necessary howeverto thoroughly mix all the components and evaporate the excess nitricacid. As with any process using acids, the materials of constructionmust be properly selected to prevent corrosion. In addition to routinesafety practices, sufficient ventilation and treatment of the acid vaporis important.

[0063] After forming a wet paste as described above, several methods canbe used to form granules. The paste can be placed in a screw-feedextruder with holes of desired diameter and then chopped into desiredlengths. An oscillating granulator may also be used to form granules ofdesired size. The material should be kept wet through all the processingsteps to minimize safety problems. The final granules can be dried inambient pressure or under vacuum. It is most preferred to dry thematerial at about 30° C. under a −12 psig vacuum. Example 15 illustratesthe process.

EXAMPLE 15

[0064] 100 ml of concentrated nitric acid (15.9M, Reagent Grade fromAldrich) was added to a glass-lined, stirred, and jacketed vessel andcooled to 0° C. 100 g of dry 5AT (Nippon Carbide), 58 g of dry AN(Aldrich ACS Grade), and 6.5 g of dry KN (Aldrich ACS Grade) were thenadded to form a slurry in nitric acid. As the mixture was stirred, theexcess nitric acid evaporated, leaving a doughy paste consisting of ahomogeneous mixture of 174 g 5AT nitrate, 64.5 g PSAN10, and a smallamount of nitric acid. This material was then passed through alow-pressure extruder to form long ‘noodles’ that were consequentlychopped to from cylindrical granules. These granules were then placed ina vacuum oven at 30° C. and −12 psig vacuum overnight. After drying, thegranules were screened and those that passed through a No. 4 mesh screenbut not through a No. 20 mesh screen were retained.

[0065] A preferred method of formulating gas generant compositionscontaining 5-aminotetrazole nitrate and phase stabilized ammoniumnitrate is described in Example 16. One of ordinary skill will readilyappreciate that the following description merely illustrates, but doesnot limit, mixing of the constituents in the exact amounts ofingredients described. For example, other oxidizers may be used in lieuof PSAN.

EXAMPLE 16

[0066] 100 ml of 70 wt. % HNO₃ solution equals 99.4 g (1.58 mol) HNO₃plus 42.6 g (2.36 mol) H₂O. The solution is mixed by stirring in 100 gdry 5-aminotetrazole (5-AT) which equals 1.18 mol 5-AT, 58 g dryammonium nitrate (AN), and 6.5 g potassium nitrate (KN) (10% of totalAN+KN). The sequence of addition is not critical. As mixing occurs, 5-ATis converted into a nitric acid salt: 5-AT(1.18 mol=100 g)+HNO₃ (1.18mol=74.4 g)=5-AT.HNO₃. The AN and KN dissolve in the water present.Excess HNO₃ (99.4 g−74.4 g=25 g) and H₂O (42.6 g) evaporate as themixture is stirred. As this occurs, AN (58 g) and KN(6.5 g)coprecipitate to form PSAN10 (64.5 g). Meanwhile, the 5-AT. HNO₃ formedwhile mixing is intimately mixed with the PSAN10. After mixing iscomplete, the end result is an intimate mixture of 174 g of5-AT.HNO₃+64.5 g PSAN10 with a small amount of HNO₃ and H₂O to keep themixture in a doughy or pasty form. Although potassium nitrate has beenused to stabilize the ammonium nitrate, one of ordinary skill willreadily appreciate that the ammonium nitrate may also be stabilized withother known stabilizers such as, but not limited to, potassiumperchlorate and other potassium salts.

[0067] Granules or pellets are then formed from the paste by methodswell known in the art. The granules or pellets are then dried to removeany residual HNO₃ and H₂O. The end product consists of dry granules orpellets of a composition containing about 73 wt. % 5-AT.HNO₃+27 wt. %PSAN10.

[0068] One of ordinary skill in the art will readily appreciate that thevarious amounts of the constituents described above can be varied toalter the combustion and ballistic properties of the gas generantcompositions.

[0069] Although the components of the present invention have beendescribed in their anhydrous form, it will be understood that theteachings herein encompass the hydrated forms as well. While theforegoing examples illustrate and describe the use of the presentinvention, they are not intended to limit the invention as disclosed incertain preferred embodiments herein. Therefore, variations andmodifications commensurate with the above teachings and the skill and/orknowledge of the relevant art, are within the scope of the presentinvention.

We claim:
 1. A gas generant composition formed from a method comprisingthe steps of: adding a nitratable base fuel to an excess amount ofnitric acid thereby forming a mixture; adding at least one oxidizer tothe mixture; stirring the mixture to form a homogeneous wet paste;forming the paste into a desired shape; and drying the formed paste. 2.The composition of claim 1 formed from the method further comprising thestep of chilling the nitric acid to 0-20° C. prior to the addition ofthe nitratable base fuel.
 3. The composition of claim 1 wherein thenitratable base fuel is selected from the group consisting of nitrourea,5-aminotetrazole, diaminotriazole, urea, azodicarbonamide,hydrazodicarbonamide, semicarbazide, carbohydrazide, biuret,3,5-diamino-1,2,4-triazole, dicyandiamide, and 3-amino-1,2,4-triazole.4. The composition of claim 1 wherein at least one oxidizer is selectedfrom the group consisting of nonmetal, alkali metal, and alkaline earthmetal nitrates, nitrites, perchlorates, chlorates, and chlorites, andalkali, alkaline earth, and transitional metal oxides.
 5. Thecomposition of claim 3 wherein at least one oxidizer is selected fromthe group consisting of phase stabilized ammonium nitrate, ammoniumnitrate, potassium nitrate, and strontium nitrate.
 6. The composition ofclaim 1 wherein the excess nitric acid has a molarity of 15.9M or less.7. The composition of claim 1 formed from the method further comprisingforming the paste into granules, pellets, or cylindrical noodles.